What is the focal spot size of a clinical linac typically?

In clinical linear accelerators (linacs), the focal spot size refers to the size of the beam of radiation that is produced by the machine at the point of treatment. A focal spot size of approximately 3 mm is commonly accepted in clinical settings, as it strikes a balance between accuracy in dose delivery and the ability to adequately cover the target tumor volume while minimizing exposure to surrounding healthy tissues.

A 3 mm focal spot helps to ensure sufficient resolution for delivering precise treatment, which is necessary for targeting tumors accurately. Smaller focal spot sizes can improve resolution and potentially reduce margin errors, but they often come with increased demands on the system and may not be necessary for all clinical applications. In contrast, larger focal spot sizes could lead to a broader beam, resulting in less precision and higher doses to adjacent healthy tissues. Thus, 3 mm is often found to be optimal for both performance and treatment efficacy in most clinical scenarios.